This invention relates to a method of manufacturing a fluid pressure servomotor having a diaphragm assembly which separates the interior of a first shell from the interior of a second shell to define first and second chambers. The peripheral surface of the diaphragm assembly is held between the first and second shells by a connection to form a unitary sealed structure. Thereafter, air is evacuated from the first and second chambers to vacuum suspend the diaphragm assembly and allow a return spring to hold the diaphragm assembly against the first shell. In response to an input force applied through an input push rod to the diaphragm assembly, fluid communication between the first and second chambers is interrupted and air is allowed to enter the first chamber to create a corresponding pressure differential. This corresponding pressure differential acts on the diaphragm assembly to provide an output member that extends through the second shell with an output force.
In order to assure that the pressure differential that is developed across the diaphragm assembly is not reduced through the communication of air by way of a leak path through a sealing surface in the unitary structure, U.S. Pat. No. 3,158,930 discloses a method of manufacturing a servomotor wherein a compressive force is continually maintained on the peripheral surface of the diaphragm since that sealing surface possesses the largest surface area and thus the greatest leak path potential. During assembly of the servomotor disclosed in U.S. Pat. No. 3,158,930, the compressive force is applied to the first and second shells causing the peripheral surface of the diaphragm to be compressed. Thereafter, a lancing operation forms a radial clip in one shell which engages a lip of the other shell to affix the first and second shells together. When the compressive force is removed, the internal resiliency of the diaphragm acts on the first and second shells to form a sealed surface that prevents the communication of air into the first and second chambers. Unfortunately during the assembly of mass produced components, the first and second shells do not always exactly match. If the dimensional tolerances of the first and second shells approach opposite manufacturing limits, it is possible that a leak path may develop and allow air to be communicated to the first and second chambers since the expansion of the diaphragm to its original shape may not be sufficient to develop a seal between the peripheral surfaces of the first and second shells. Later, when air is evacuated from the first and second chambers, air flow through the leak path reduces the effective pressure differential as measured by the output force of the output push rod. Unfortunately since the first and second shells have been lanced together, if an attempt is made to salvage the components that make up the servomotor, the shells and diaphragm are often destroyed even though the non-conformity of only one component alone resulted in the leak path. Thus, corrective action to ascertain which component does not confirm to the manufacturing specification is often time consuming since additional damage can be caused by the salvage operation.